1. Field of the Invention
The present invention relates to an information reproducing device, and more particularly to an information reproducing device and an information reproducing method which perform decoding processing by using a PRML (Partial Response and Maximum Likelihood) method.
2. Description of the Related Art
Recently, there has been widely spread an information recording/reproducing device that performs a recording and reproducing process to a storage medium, e.g., an optical disk such as a DVD (Digital Versatile Disc), and the higher recording density with various methods has been demanded. With respect to this, for example, as a method of recording and reproducing process of an optical disk or the like, there is a PRML method.
That is, if the recording density becomes higher at the time of recording/reproducing information on a storage medium, a mark with a desired size can not be recorded due to an affect of the thermal interference when recording a shortest mark or a shortest space in a recording mode. Further, in reproduction, since the recorded shortest mark or shortest space is small, the waveform interference further affects, and an amplitude corresponding to the shortest mark/space becomes consequently small. That is, the shortest mark/space which is smaller than the desired size is formed in recording, and the amplitude corresponding to the shortest mark becomes small due to the affect of the waveform interference in the reproduction mode.
On the other hand, in a Viterbi algorithm which is the PRML method, the linearity is not necessarily demanded to the shortest mark/shortest space. Giving explanation taking an RLL (1, k) modulation method as an example, the shortest mark/shortest space is 2T in this modulation method. 1T is not allowed in a state transition diagram of FIG. 6 of the Viterbi algorithm determined by the modulation method. That is, a 2T signal level does not necessarily have to match with a 2T ideal signal level, and allows identification.
As a cited reference disclosing this PRML method, there is Jpn. Pat. Appln. KOKAI Publication No. 6-4810, in which the convolution integration of a decoding result and a PR (Partial Response) class is carried out in order to obtain an ideal waveform, an equalization error between the ideal waveform and an equalizer output is calculated and reproduction signal processing parameters (for example, a tap coefficient, a gain, an offset and the like of an equalizer) are changed by using an LMS (Least Mean Square) algorithm so as to minimize the equalization error.
When the recording density is improved, however, the shortest mark/space is generally hard to be formed, and in fact the shortest mark/space is formed with a smaller length than the desired length. According to the state transition diagram of a Viterbi decoder in FIG. 6, however, since a code of a length not more than the shortest mark/space is not admitted, a decoding error hardly occurs even if a value of a level corresponding to the shortest mark/space among levels of an actual reproduction signal is shifted in a direction to reduce the amplitude as compared with the ideal waveform.
Thus, as the recording density is increased, for example, when a tap coefficient of the equalizer is calculated by using the LMS algorithm so as to minimize the above-described equalization error, there is disadvantageously outputted an equalization coefficient amplified on a higher frequency side than the equalization coefficient of the equalizer which is required in the Viterbi decoder. As a result, the noise is amplified more than necessary, and the decoding error can not be consequently satisfactorily reduced.
That is, in this prior art, even if the recording density is increased and the amplitude of a response waveform corresponding to the shortest mark/shortest space becomes small, there is generated a tap coefficient of an FIR (Finite Impulse Response) filter having an equalizer characteristic which forcibly increases the gain on the high frequency side so as to increase the amplitude of the shortest mark/shortest space after equalization.
As a result, since equalization close to the 2T signal level of the ideal waveform is performed beyond the 2T signal level which is required in the Viterbi decoding processing, there is generated the equalization coefficient having the characteristic of the gain larger than that required in the Viterbi decoding processing in a high frequency band. There is, therefore, a problem that the noise is unnecessarily amplified and the decoding error can not be satisfactorily reduced.